The pancreas is a complex organ consisting of distinct exocrine (acinar cells), endocrine (islet cells), and duct cell compartments that together regulate aspects of digestion and glucose homeostasis. Alterations in pancreatic cell function lead to several critical human diseases, including diabetes and two diseases of the exocrine pancreas - pancreatitis and pancreatic cancer. Recent studies have shown that acinar cells are intimately involved in these exocrine pathologies, yet our knowledge of the regulatory mechanisms controlling the stability of the acinar cell phenotype during disease progression remains incomplete. Studies on Mist1, a basic helix-loop-helix transcription factor that is expressed in acinar cells, have provided the first insight into the molecular processes responsible for establishing cell polarity, protein synthesis functions, and maintenance of the acinar cell lineage. Pancreata from Mist1 null (Mist1KO) mice are characterized by highly disorganized acinar cells that show defects in EGFR signaling and in the pathways controlling apical-basal polarity, regulated exocytosis, ER protein processing and intercellular communication. Mist1KO acinar cells also are primed to undergo acinar-to-ductal conversion, revealing a unique potential for plasticity in the absence of Mist1. The Mist1KO model is of particular biomedical significance because similar pancreatic defects are associated with human pancreatitis and pancreatic cancer, yet a detailed understanding of the transcriptional networks involved in these diseases remains elusive. The main objective of this renewal is to position Mist1 within regulatory pathways that direct exocrine function and stability in both mouse and human pancreata. Specifically, we will (i) define and characterize Mist1 gene targets and transcription complexes using inducible transgenic lines and mass spectrometry approaches and evaluate the importance of these gene products to the pathogenesis of human disease, (ii) establish how Mist1 affects acinar cell regeneration and cellular plasticity events during pancreatitis and in novel 3D culture models, and (iii) position Mist1 within the unfolded protein response (UPR) pathway via acinar-restricted deletion of the Xbp1 gene. Successful completion of these studies will define the molecular basis of the Mist1KO pancreas defects, identify key Mist1 target genes, establish how Mist1 prevents acinar-ductal metaplasia, and assess the role of Mist1 within the UPR. These studies also will evaluate if loss of Mist1 is associated with the earliest stages of human exocrine pancreas disease. Identifying these key nodal points is critical to our future goals of developing new therapeutic approaches to reprogram aberrant cells during disease progression.